Hi hiddenspring,
The first thing I would consider is that ILD is not exact so a layer that is rated at a specific ILD may not be exactly the same as another that has the same rating (see post #6 here). This is just the natural variance of the production process.
Outside of the variance in ILD in each layer … ILD differences in a range of 2 or 3 (or sometimes more) wouldn’t be detectable for most people.
ILD is also only one of the specs that leads to the perception of softness/firmness so blended and 100% natural Talalay that have the same ILD may feel different because they have a different density and compression modulus.
The Savvy Rest Talalay is 100% natural (they don’t offer blended as an option). I would also remember though that all the layers in a mattress interact with every other layer so it would only be “safe” to make an assumption that a slightly lower ILD would feel softer if the other two layers were identical. A softer middle layer would affect the feel of the top layer and it would feel softer than another mattress with a firmer middle layer even if the top layer was the same. If the other layers are “close” though … then it’s likely that the softer top layer would be also be too soft based on your feedback. The latex supreme is a different construction with different layering so that alone would make a difference even if the top layer was the same but in addition to the differences in the design the top layer in the Supreme is also a different thickness and is also convoluted so it wouldn’t be directly comparable to a 3" layer of solid Talalay (even if the layers below it were the same). In addition to this, OMF puts their latex mattress on an “active” box spring which will also change the feel of the mattress compared to the same mattress that is tested on a solid foundation that doesn’t flex. You can read more about the various factors that contribute to the “softness” of an individual layer in post #2 here.
Yes … a firmer top layer would create a more firm sleeping surface and the other way to increase the firmness is to decrease the thickness of the soft top layer so it is closer to the firmer layers below it (you can read more about the effect of thickness on the softness and response of a mattress in post #14 here). If a mattress is too firm then adding a topper in a suitable thickness and softness is certainly a valid way to increase the softness of the comfort layers yes but the key is to choose the thickness and firmness of the topper based on your actual experience after you have slept on the mattress instead of “theory” (there are some topper guidelines in post #2 here)
A two layer mattress will be different from a 3 layer mattress (or any thicker latex mattress) even if the top two layers in both are the same. Dunlop has a higher compression modulus than Talalay so it “feels” firmer with deeper compression (past 25%) than Talalay but it will also be softer with shallower compression that is less than 25%. Both come in a range of firmness levels and both types of latex in the “right” layering can provide good pressure relief and good alignment in a suitably designed mattress. “Alignment” (how evenly you sink in to a mattress) is always the goal and “support” or firmness is just a means to an end. The choice between Dunlop and Talalay would be a preference rather than a “better worse” choice both in the comfort layers and the support layers because both are available in softer and firmer versions. You can read a little more about the difference between them in post #7 here.
If you are seriously considering a 6" mattress with two layers then I would specifically test this (Savvy Rest has this option available) because it may feel significantly different from a thicker mattress with 3 layers (although you are both lighter and would compress the mattress less so the differences may be less for you). With your lighter weight 6" may work well but it will certainly be “different” from the mattresses you have tested.
If you have personal experience with a specific layering then smaller changes in firmness levels, layer thickness, layer combinations are easier to predict but multiple differences or larger changes in design (such as removing a complete layer) will have a compounding effect and make comparisons more difficult. In most cases smaller differences in design aren’t that important to most people and are well within the wider “range” that works well for them but for those who are more sensitive to smaller differences in two mattresses or if the differences are more a single small difference then it can result in two mattresses that can feel quite different for some people (even though others may not notice any significant difference at all).
For the most part and for most people … close approximations to something you have tested would be “close enough” and trying to make comparisons that are more finely detailed or “granular” than is realistically possible (or trying to predict the “feel” of a mattress that is significantly different from what you have tested) can be an exercise in frustration because of all the many variables involved.
Phoenix